The design of small electric motors such as may be used in power tools receives a very great amount of attention. In particular attempts are made to facilitate assembly processes that are carried out in the factor and to facilitate subsequent maintenance of the motor. The most common maintenance operation is that of brush replacement.
In order to facilitate the assembly of a motor it is desirable to automate the assembly process so far as possible. While parts of the process are relatively easy to automate other parts are not.
A factor that influences the ease of assembly of an electric motor is the number of electrical connections that have to be made during assembly and the ease with which these connections can be made. It is known to use strip conductors that have sufficient stiffness that they return their shape during normal use and this greatly facilitates their connection during motor assembly. Sometimes the same basic electric motor may be used in a variety of devices and, according to its use, impedances are connected in series or in parallel with a part of the motor circuit; in such cases it has not previously been possible to provide a satisfactory arrangement employing stiff conductors yet also allowing for connection of these impedances when approporiate.
In order to simplify assembly of products incorporating electric motors, it is known to provide a motor unit which can be installed in a tool as a single module. This also allows for the whole motor to be replaced readily in the event that a component of the motor fails; while such an operation can be relatively simple, it provides a rather drastic solution to the common problem of, for example, a simple brush wearing out. It is also known to provide a modular motor of this kind which can readily be dissembled into component parts comprising an armature, a stator assembly and a pair of end caps on opposite ends of the stator assembly, the end cap at the commutator end of the motor carrying strip connectors and brush boxes which in the assembled tool make the appropriate electrical connections with the brushes and field coils. In this known design the armature has to be located in position before the brush boxes and brushes are installed since otherwise the brushes in the brush boxes would interfere with insertion of the armature. Thus in assembling this motor the end caps and stator assembly are secured together with the armature in position before the brush boxes and brushes are installed; the brush boxes and brushes have to be installed as a separate operation at the end.
It is of course important that the electrical connections between the components of a motor are reliable and able to withstand the vibration to which they are inevitably exposed in use. In a conventional arrangement for making an electrical connection to a brush holder, the brush holder is provided with a tag over which a lead is clipped. Unless special precautions are taken, it is possible for the lead to become detached from the tag.
With regard to brush replacement, a known method which avoids the need to remove the brush box involves bending open the rear end of the brush box to gain access to the brush and spring. While this arrangement will work once or twice, repeated bending of the rear end of the brush box leads eventually to the end breaking off. One solution to this problem that has been proposed is to make the rear end of the brush box replaceable, but of course this complicates the design of the brush box.
A further difficulty encountered by designers of electric motor relates to the location of the impedance (capacitor) that is commonly connected across the power supply leads to the motor. In many respects it is convenient to mount the impedance on the motor itself but as this part vibrates during operation of the motor, there is a risk of the electrical connections to the impedance breaking.